Comparison of the surface brightness of some possible HeH⁺ pure rotational transitions in the dense (upper panel) and diffuse (lower panel) parts of RS Oph calculated using CLOUDY, considering HeH⁺ collisional data based on EMAA data (indigo), CB data considering only Δυ = 0, ±1, ∆ J = ±1 (cyan), and CB data considering both Δυ = 0, ±1, ∆J = ±1, and Δυ = 0, ∆J = ±2 (blue). The plotted CB data considering Δυ = 0, ±1, ∆ J = ±1, and Δυ = 0, ∆ J = ±2 are summarized in Table 3.